In order to better understand the invention, we will first of all briefly describe the closest state of the art, to the best of the Applicant's knowledge, with reference to FIGS. 1 and 2 which show, schematically, in plan and cross-sectional views a control of coupling mechanism in the position where the chronograph is working. As can be seen in FIG. 1, the coupling device is actuated by a circular cam 1 having, on the exterior, a drive toothing 3 on which a manoeuvring lever, actuated by a push button (not shown), acts, said cam 1 being held in a determined position by means of a jumper spring 5. Cam 1 includes, on the inside, a second control toothing 2, whose teeth separating two successive hollows 4 of the toothing are truncated and have a flat 6. Outer toothing 3 includes twice as many teeth as inner toothing 2, such that, when driven through one step via actuation of a push button, a hollow 4 and a flat 6 are brought successively opposite a determined reference position. The control of coupling mechanism 10 is formed by a lever basically having three concurrent arms. A first arm 11 at its end pivoting at 12 in plate 7 or in a mechanism plate 8. A second arm 13 has at its end a tooth 14a held resting on a flat 6 of inner control toothing 2 by a return spring 9 stopped on a portion 14 bent into a Z shape. A third flexible arm 15 has at its end a fork 16, preceded by a V shaped fold 18 having a plane 18a inclined against an edge 8a of plate 8. In the coupled position shown in FIG. 2, coupling wheel 17 is driven, via the action of an annular leaf spring 21, by friction by the intermediate second wheel 19. When on wishes to stop the chronograph, an application of pressure on the push button causes cam 1 to rotate by a half-step and tooth 14a falls into a hollow 6, which causes inclined plane 18a to slide against edge 8a of plate 8 to a position in which V-shaped fold 18 is on plate 8, which allows fork 16 to disconnect coupling wheel 17 from intermediate second wheel 19 by compressing spring 21. In order for the coupling mechanism that has just been described to operate properly, it is thus necessary to be able to adjust precisely all the mechanical and physical parameters such as the tolerances as regards cam 1, control device 10 and the elasticity modules relating to return spring 9, arm 15 of control device 10 and annular spring 21.
With this device, a blockage of control device 10 is often observed during return to the stop or zero chronograph position, which can only be removed or reduced by providing elasticity modules, such that greater pressure has to be applied It will also be observed that in the stop or zero chronograph position, which is the position occupied for most of the time, flexible arm 15 is permanently stressed, which can obviously cause, at least partially, deformation that is detrimental to its elastic properties.